The virus was first detected about two weeks ago by the military's Host-Based Security System, but it had not halted missions flown remotely over Afghanistan and other warzones from Nevada's Creech Air Force Base, Wired said.

No classified information was believed to have been lost or sent outside the network, though the resilient virus resisted several attempts to remove it.
"We keep wiping it off, and it keeps coming back," a source familiar with the network infection told the US magazine.

"We think it's benign. But we just don't know."

Military network security specialists said it remained unclear whether the virus was intentional and how far it had spread, but they were certain it had infected Creech's classified and unclassified machines. Secret data may thus have leaked out and reached someone outside military officials.

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The US military does not hide its own drone flights in Libya or Afghanistan, in contrast to the CIA's covert missions to take out al-Qaeda extremists in Pakistan, Yemen and elsewhere.

The drones have become a critical weapon of choice for the United States in fighting militants abroad.

In Pakistan alone, around 30 drone strikes have been reported since elite US forces killed al-Qaeda chief Osama bin Laden on May 2 near the country's main military academy in Abbottabad, close to the capital.

The virus is believed to have spread at Creech through removable hard drives used to load map updates and transfer mission videos from one computer to another, Wired said.

Drone units at other US Air Force bases around the world have now been told to stop using them.